Recovery of sodium persulfate solutions

ABSTRACT

SODIUM PERSULFATE WHICH IS RESUABLE FOR ETCHING IS RECOVERED FROM A SPENT ETCHING SOLUTION BY LOWERING CONCENTRATIONS OF SODIUM SULFATE AND METAL IN THE SOLUTION TO ACCEPTABLE LEVELS. THE METAL SULFATE CONCENTRATION IS LOWERED BY ELECTROLYSIS THEREBY PLATING THE METAL FROM THE SOLUTION AND GENERATING SULFURIC ACID. THE SULFURIC ACID IS CONVERTED TO SODIUM SULFATE BY THE ADDITION OF SODIUM HYDROXIDE BY COOLING THE SOLUTION TO -5 TO +5 SULFATE IS LOWERED BY COOLING THE SOLUTION TO -5 TO +5 DEGREES CENTIGRADE THEREBY PRECIPITATING SODIUM SULFATE FROM THE SOLUTION. TO LOWER THE ACTIVITY OF THE PERSULFATE DURING PLATING, THE SOLUTION IS ADVANTAGEOUSLY MAINTAINED AT -5 TO +5 DEGREES CENTIGRADE DURING PLATING THEREBY PRECIPITATING THE SODIUM SULFATE IN TWO STEPS.

Feb. 12, 1974 v R,.. @WENS 3,1919% RECOVERY OF SODIUM PER SULFATE SOLUTIONS Filed Jan. 16, 1973 "United States Patent US. Cl. 204-82 6 Claims ABSTRACT OF THE DISCLOSURE Sodium persulfate which is reusable for etching is recovered from a spent etching solution by lowering concentrations of sodium sulfate and metal sulfate in the solution to acceptable levels. The metal sulfate concentration is lowered by electrolysis thereby plating the metal from the solution and generating sulfuric acid. The sulfuric acid is converted to sodium sulfate by the addition of sodium hydroxide and the concentration of sodium sulfate is lowered by cooling the solution to to +5 degrees centigrade thereby precipitating sodium sulfate from the solution. To lower the activity of the persulfate during plating, the solution is advantageously maintained at 5 to +5 degrees centigrade during plating thereby precipitating the sodium sulfate in two steps.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to aqueous sodium persulfate etching solutions, and more particularly, to the recovery of a sodium persulfate solution from a spent sodium persulfate etching solution containing sodium sulfate, metal sulfate and sodium persulfate.

(2) Description of the prior art Peracids, such as ammonium persulfate, are commonly employed to etch metals such as copper, cobalt, iron, magnesium, nickel, zinc and alloys thereof. For example, in

the manufacture of printed circuits, a peracid may be used to etch away unwanted metal through an etch resist to form a desired circuit pattern. The ammonium persulfate removes the metal as a metal sulfate and converts the ammonium persulfate to ammonium sulfate. With continued use of the peracid, the persulfate values decrease until the concentration is too low for a satisfactory etch rate or quality of etch. At this point, the peracid is spent and is replaced with a fresh etching solution.

A problem of increasing importance is what to do with the spent etching solution. In the prior art, an ammonium persulfate solution is recovered from a spent ammonium persulfate solution by cooling the spent solution to a temperature of 0 to 20 degrees centigrade thereby percipitating out a double salt of ammonium sulfate and metal sulfate to leave a solution of ammonium persulfate. Although this permits the recovery of an ammonium persulfate solution from the spent etching solution, it still leaves the problem of disposing of the double salt.

It is also known to use a spent ammonium persulfate etching solution as an anolyte and to separate the anolyte from a catholyte, such as a solution of ammonium sulfate and sulfuric acid with a cation exchange membrane. The cation exchange membrane permits metal ions to enter the catholyte from the anolyte while inhibiting persulfate ions from entering the catholyte. As a result, metal ions are plated from the catholyte while sulfate values in the anolyte are oxidized to persulfate thereby regenerating the ammonium persulfate etching solution. Alternatively, a portion of the ammonium sulfate and metal sulfate is precipitated by cooling the solution to a temperature of from 0 to 20 degrees centigrade leaving a solution of "ice ammonium persulfate containing in addition ammonium sulfate and the metal sulfate. The persulfate free precipitate is reconstituted and used as a catholyte and the ammonium persulfate solution is used as an anolyte. A cation membrane is used which inhibits the interchange of ions other than hydrogen ions to separate the anolyte and catholyte. However, the ditficulty with this process is that the cation membrane is not efiicient and does permit persulfate values into the catholyte thereby interferring with the plating of the metal. Also, the cation membrane is failure prone.

An additional difliculty with the prior art is that ammonium ions are very difiicult to keep out of waste treatment systems as they are highly soluble, and are diflicult to remove due to the many soluble complex compounds which they form with other materials.

SUMMARY OF THE INVENTION This invention contemplates a method of recovering a solution of sodium persulfate from a spent sodium persulfate etching solution containing sodium sulfate, a metal sulfate and sodium persulfate and includes the steps of (1) plating the metal from solution while generating sulfuric acid, (2) adding sodium hydroxide to the solution to convert the sulfuric acid to sodium sulfate and (3) precipitating sodium sulfate from the solution.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, a schematic of an electrolytic cell is shown which is suitable for practicing the method of this invention.

DETAILED DESCRIPTION According to the method of this invention, a sodium persulfate solution is recovered from a spent aqueous etching solution of sodium persulfate containing sodium sulfate, metal sulfate and sodium persulfate by removing the sodium sulfate from solution and plating the metal from the metal sulfate from solution to leave a sodium persulfate solution.

Sodium sulfate is removed from a spent aqueous etching solution of sodium persulfate by cooling the spent solution to a temperature of from -5 to ;+5 degrees centigrade thereby precipitating the sodium sulfate from the solution. For example, 40 grams per liter of sodium sulfate are soluble in water at zero degrees centigrade. Accordingly, any sodium sulfate in the spent solution which exceeds 40 grams per liter is precipitated by cooling the spent solution to zero degrees centigrade. This permits a spent solution to be reused (after desired sodium persulfate concentrations are obtained) time and time again without excessive concentrations of sodium sulfate building up in the solution. On the other hand, 500 grams per liter of sodium persulfate are soluble in water at zero degrees centigrade. As a typical spent etching solution has a molar concentration of sodium persulfate of approximately 0.89 (211 grams per liter), no sodium persulfate is lost from solution during the precipitation of the sodium sulfate. In addition, 144 grams per liter of cupric sulfate are soluble in water at zero degrees centigrade. As a typical spent etching solution has a molar concentration of cupric sulfate of 0.35 (56 grams per liter), there is also no cupric sulfate precipitated from solution during the precipitation of the sodium sulfate.

The precipitated sodium sulfate can be pumped from the solution as a slurry leaving sodium persulfate, metal sulfate and 40 grams per liter of residual sodium sulfate in solution. The reaction for removing sodium sulfate from solution is as follows:

cool X804 NazSgO Na SO X Nfl SgO; Na SO4 l The metal from the metal sulfate is removed from the solution by electrolysis, and the sulfate from the metal sulfate is removed by precipitating the sulfate from solution as sodium sulfate. As will be appreciated by one skilled in the art, sodium persulfate is a suitable etchant for metals, such as copper, cobalt, iron, magnesium, nickel, zinc and alloys thereof. Accordingly, the metal sulfate may be the sulfate of anyone of these metals or alloys thereof. As will also be appreciated, each of these metal sulfates and their alloys may be reduced by electrolysis thereby plating the metal onto a cathode of an electrolytic cell. Cupric sulfate is given only by way of example, and the particular metal sulfate in the solution is in no way critical to the process. If, for example, cupric sulfate is the metal to be removed from solution by electrolysis, a cathode constructed of 316 stainless steel and a current density of 30-50 amps per square foot is suitable for removable of the copper. An anode constructed of platinum plated titanium and a current density of 30-50 amps per square foot is suitable for the oxidation at the anode to sulfuric acid of the sulfate ion generated by the reduction of the copper at the cathode.

When a continuously circulating etchant is employed, i.e., spent etchant is continuously being removed from an etching tank (not shown) and new etchant is continuously being added, it is not necessary or even desirable to removable all of the metal from the spent etchant before recirculating the etchant to the etching tank. It is only necessary that the metal concentration in the etching solution be maintained at a level which will permit etching at the required speed and with the required etch quality. For example, three ounces of copper per gallon (22.5 grams per liter) can be readily tolerated in an etchant without harmful effect. Accordingly, when a fresh etchant is used, circulation is initiated when approximately 22.5 grams per liter of copper (56 grams per liter of cupric sulfate) builds up in the etching tank. By removing only 7.5 grams per liter of copper from the etchant prior to returning the etchant to the etching tank, the etchant in the tank is maintained below the 22.5 grams per liter of copper and optimum etching is facilitated without the necessity of removing all of the copper. However, in a batch type process,, i.e., the etchant is not circulated and is used until it is no longer an effective etchant, it is desirable to remove as much copper as possible from the etchant prior to reuse of the etchant. After removal of the metal from the solution by electrolysis, sodium persulfate, sulfuric acid, residual sodium sulfate and residual cupric sulfate are left in solution. The reaction for removing the metal from the solution is as follows:

(2) elect.

X304 N32S203 211+ xi H2804 Nazszog As the cathode is exposed to the sodium persulfate during the plating of the metal thereon, the sodium persulfate attacks the metal plated onto the cathode. However, the metal is readily plated onto the cathode at a much greater rate than the sodium persulfate attacks the metal thereby permitting, if so desired, the removal of essentially all of the metal from the solution. Also, as sodium persulfate etching solutions are generally used for etching at elevated temperatures, e.g.,, 49 degrees centigrade, a solution which is a spent solution at elevated temperatures has essentially no etching activity at lowered temperatures, e.g., zero degrees centigrade. For example, it has been found that plating copper at zero degrees centigrade in a 0.89 molar solution of sodium persulfate results in a reduction of the molar concentration of the sodium persulfate to 0.78, i.e., 26 grams per liter of sodium persulfate are lost. As a result, the amount of persulfate recovered is maximized thereby reducing the amount of sodium persulfate which must be added to bring the recovered sodium persulfate solution back to full etching strength. This is to be contrasted with prior art processes which generate persulfate values at the anode and require the cathode to be protected from the persulfate to prevent undesirably large quantities of metal from being etched from the cathode.

The sulfuric acid generated during electrolysis is converted to sodium sulfate by adding stoichiometric amounts of sodium hydroxide to the solution and the sodium sulfate is precipitated by maintaining the solution at a temperature of from 5 to +5 degrees centigrade. As discussed above, the sodium sulfate is removed as a slurry thereby leaving sodium persulfate with residual amounts of sodium sulfate and metal sulfate in the solution. For example, if the spent sodium persulfate solution having the initial sodium sulfate precipitated therefrom is placed in an electrolytic cell and the metal, e.g., 7.5 grams per liter of copper, is electroplated onto a cathode, a solution having a molar concentration of 0.78 sodium persulfate and 0.12 sulfuric acid is generated. As noted above, there is a slight drop in the molar concentration of the sodium persulfate. The addition of 9.8 grams per liter of sodium hydroxide converts the sulfuric acid to 17.3 grams per liter of sodium sulfate which is precipitated at a temperature of zero degrees centigrade. The resulting solution of sodium persulfate can then be reused as an etchant merely by adding sufficient sodium persulfate to obtain a desired concentration. The reaction for converting the sulfuric acid and precipitating the sodium sulfate is as follows:

cool NazSzOa H2804 ZNaOH NazSzOa NazSO l, 21110 As will be appreciated, the sodium sulfate can be precipitated from the solution as a single step. In other words, rather than precipitate sodium sulfate from the solution prior to electrolysis and then again afterwards, the sodium sulfate may be precipitated in a single step after electrolysis. The reaction steps are then as follows:

elect. X504 Ntlzszoli Na1SO 211+ X1. H2804 NazS2Oa NazSOt cool HzSO; Nazszoa Nazsor NaOH Nmszos 2N82SO J, 21120 Refering now to the drawing, a sodium persulfate solution is recovered from a spent sodium persulfate etching solution 11 containing 57 grams of sodium sulfate per liter, 56 grams of copper sulfate per liter and 211 grams of sodium persulfate per liter in an electrolytic cell 12. The spent etching solution 11 enters the cell by Way of conduit 13 and is cooled in the cell to a temperature of from 5 to +5 degrees centigrade by cooling coils 14-14. As a result, 17 grams of sodium sulfate (at zero degrees Centigrade) is precipitated from the solution and is pumped from the cell as a slurry 16 through conduit 17. As the sodium persulfate etching solution is normally used at elevated temperatures, e.g., 49 degrees centigrade, cooling of the spent solution in the cell may be facilitated by precooling the spent solution prior to its introduction into the cell.

After precipitation of the sodium sulfate, current is applied to the electrolytic cell to plate the metal from the metal sulfate onto cathode 18 and to generate sulfuric acid at anode 19. If 7.5 grams of copper per liter are plated onto the cathode then 12 grams per liter of sulfuric acid are generated at the anode.

After the copper is plated from solution, the electrolytic cell is deactivated and the sulfuric acid formed in the solution as a result of the electrolysis is converted to sodium sulfate by the introduction of stoichiometric amounts of sodium hydroxide in a solution 21 by way of conduit 22. The addition of 9.8 grams per liter of sodium hydroxide to the solution is sufiicient to convert the sulfuric acid to sodium sulfate. By maintaining the solution at a temperature of from 5 to +5 degrees centigrade, the sodium sulfate formed by the reaction of the sodium hydroxide with the sulfuric acid is precipitated from the solution and removed as the slurry 16 through conduit 17. This leaves sodium persulfate in solution with acceptable amounts of sodium sulfate and copper sulfate residuals which are pumped from the cell by way of conduit 23 as sodium persulfate solution 24. Maintenance of the solution at the desired temperature during the addition of the sodium hydroxide is facilitated by precooling the sodium hydroxide.

As will be appreciated, the electrolytic cell can be operated continuously rather than in the batch type operation described above. For example, the flow of spent etchant and sodium hydroxide solution into the cell and the flow of the sodium sulfate slurry and sodium persulfate solution from the cell can be metered so that spent solution is added at the same rate the sodium persulfate solution is removed, the slurry is removed at the same rate sodium sulfate is precipitated and sodium hydroxide is added at the same rate sulfuric acid is generated. In this manner, each step of the process may be carried out continuously in the same cell.

In addition, it is not necessary for all of the steps of the process to be carried out in the electrolytic cell. For example, the sodium sulfate may be precipitated from the solution prior to introducing the solution into the cell and the sodium hydroxide may be added after the solution is removed from the cel. Also, it is not essential that the cell be cooled during the plating of the metal. However, as the activity of the persulfate is reduced by cooling, it is advantageous to cool the cell during plating And, if the cell is cooled, it simplifies the process and facilitates the performance of all of the process steps in a single cell.

What is claimed is:

1. A method for recovering a sodium persulfate solution from a spent sodium persulfate etching solution containing sodium sulfate, a metal sulfate and sodium persulfate, comprising the steps of:

plating the metal from the metal sulfate onto a cathode while converting the sulfate from the metal sulfate to sulfuric acid;

adding stoichiometric amounts of sodium hydroxide to convert the sulfuric acid to sodium sulfate; and cooling the spent solution to precipitate the sodium sulfate from solution.

2. The method of claim 1 wherein the spent etching solution is cooled to a temperature of from -5 to +5 degrees centigrade prior to electrolysis to precipitate sodium sulfate from solution.

3. The method of claim 2 wherein the solution is maintained at a temperature of from -5 to +5 degrees centigrade during the plating of the metal.

4. A method of recovering a sodium persulfate solution from a spent sodium persulfate etching solution containing sodium sulfate, a metal sulfate and sodium persulfate, the method comprising:

cooling the solution to a temperature of from -5 to +5 degrees centigrade to precipitate sodium sulfate from solution;

applying an electric current to the solution to plate metal from the metal sulfate onto a cathode while converting sulfate from the metal sulfate to sulfuric acid; adding stoichiometric amounts of sodium hydroxide to convert the sulfuric acid to sodium sulfate; and

cooling the solution to a temperature of from -5 to +5 degrees centigrade to precipitate sodium sulfate from solution thereby forming a sodium persulfate solution.

5. The method of claim 4 wherein the metal sulfate is copper sulfate.

6. The method of claim 4 wherein the solution is maintained at a temperature of from -5 to +5 degrees centigrade during plating of the metal.

References Cited UNITED STATES PATENTS 1,059,809 4/ 1913 Adolph et al 204-82 2,281,090 4/ 1942 Salleras 204-82 3,399,090 8/1968 Caropress et al. 156-19 3,406,108 10/1968 Radimer et al 204-82 3,470,044 9/ 1969' Radimer 156-19 3,505,135 4/1970 Lindstrom 156-19 2,989,371 6/ 1961 Mehltretter et al. 204-82 X FOREIGN PATENTS 867,386 2/ 1953 Germany 204-82 FREDERICK C. EDMONDSON, Primary Examiner US. Cl. X.R. 156-19 

